Automatic frequency control



AUTOMATIC FREQUENCY CONTROL man@ ATTR/VEY April 4, 1950 R. RFREAS, JR2,503,105

AUTOMATIC FREQUENCY CONTROL Filed Sept. 30, 1947 2 Sheets-Sheet 2 RELAYTTRWEY.

Patented Apr. 4, 1950 AUTOMATIC FREQUENCY CONTROL Robert R. Freas, Jr.,Jermyn, Pa., assignor to Radio Corporation of America, a corporation ofDelaware Application September 30, 1947, Serial No. 777,013

(Cl. Z50-36) 7 Claims.

This invention relates to the automatic frequency control of oscillationgenerators and has for its principal object the provision of an improvedapparatus and method of operation whereby the output of a crystalcontrolled oscillation generator may be utilized (l) to regulate withcrystal accuracy the output frequency of an oscillation generator whichis not crystal controlled or (2) to regulate with crystal accuracy theoutput frequencies of an oscillation generator which is not crystalcontrolled and may be operated at a plurality of closely spaced,definite, predetermined frequencies.

In the illustrated form of the invention, these results are produced bymeans including a motor which has a rotor mechanically coupled to thefrequency control element of the generator which is to have itsfrequency controlled or regulated. This motor has four stator windingsarranged in quadrature and provided with a common terminal at one oftheir ends. To this common terminal is applied potential of a frequencyproportional to that of the regulated generator, and to the other fourterminals of the stator windings are applied potentials which have afrequency proportional to that of the crystal controlled generator andwhich are separated in phase by ninety electrical degrees.

The relation between the frequencies of the potentials applied to thedifferent terminals of the stator windings is such that the motorproduces no torque when the desired relation exists between thefrequency of the regulated generator and that of the crystal controlledgenerator.

When the frequency of the regulated generator tends to increase,however, the motor produces torque in a direction to decrease thefrequency of the regulated generator. Likewise, when the frequency ofthe regulated generator tends to decrease, the motor produces torque inthe opposite direction, thus increasing the frequency of the regulatedgenerator.

Multivibrator circuits areutilized for maintaining'the desired relation(1,) -between the frequencies of the crystal controlled generator andthe v potentials applied to the outer terminals of the stator windingsand (2)` between the frequency of the regulated generator and thepotential applied to thecoinmon terminal of the stator windings.

These multivibrator circuits each include a pair of electron dischargeelements having operating potential applied through a resistor which iscommon to their-anodes and through separate re-4 sistors which areindividualto the separate anodes. The anode of each electron dischargeelement is coupled to the grid of the other through a resistor shunted.by a capacitor and the cathodes of the two elements are groundedthrough a resistor shunted by 'a capacitor.

With these connections, current conduction is stable in one or the otherof the electron discharge elements and is transferred from one to theother of the elements in response to the application of a negative pulseat the junction between the common and separate resistors..

Because of the fact that the torque of the motor decreases rapidly withincrease in speed, it is desirable that variation in the-motor speed bemaintained within predetermined limits'. Such limits are maintained bymeans of fixed-frequency oscillation generator elements one of whichtakes control when the frequency of the regulated generator exceeds onepredetermined limit and the other of which takes control when thefrequency of the regulated generator goes beyond the other predeterminedlimit. Such control is maintained only during times when the torque ofthe motor otherwise would be lnsucient to apply the required correctiveeffect to the frequency control element of the regulated generator. Atthe ends of such times, controlis returned automatically to the outputfrequency of the regulated generator.

Important objects of the invention are the provision of an improvedfrequency control system which is capable of affording a large number ofcrystal controlled frequencies at given channel spacing from a regulatedoscillation generator; the provision of an improved circuit forregulating the output frequency of an oscillation generator which is notcrystal controlled; and the provision of an improved means formaintaining the speed of a control motor within such limits as toproduce a torque greater than a predetermined minimum. I l

The invention will be better `understood from the following description,considered in connection with theA accompanying drawings, and its scopeis indicated by the appended claims.

Referring to the drawings: l I I l Fig. 1v is a schematic dagramshowingthe re` lation between the various elementsof the invention whicharedepicted in the form of boxes bearing explanatory legends, and

Fig. 2 is a wiringdiag'ram of the frequency controlsystem of Fig. .1;

The automatic-frequency control apparatus of Fig. l includes 'a -crystalcontrolled oscillation generator `|`0 which delivers its outputthroughafixed frequency divider I I to a multivibrator circuit or frequencydivider MV1. Connected to the two opposite anode leads of the dividerMV1 are similar frequency dividers MVz and MVS. The anode leads of thedivider MVz are connected through amplifiers I2 and I3 to the outsideterminals of the field windings I4 and I5 of an induction motor IE. Theanode terminals of the divider MV1; aresimilarly connected through.amplifiers II and I8 to the eld windings I9 and 2D of the motor I6.Amplifiers I2, I3, II and I8- may each contain a triode amplifiersection and a thyratron power amplier section.

With these connections, there are applied to the terminals of thewindings I4, I5', I9 and 20 potentials which (l) are displaced in phaseby ninety electrical degrees and (2) assume their maximum values atsuccessive instants of time.

The motor I6 includes a rotor 2l which is rnechanically coupled througha shaft 22 (shown as a broken line) to an oscillation generator 23 whichis to have its frequency regulated.

The generator 23 delivers its output through a. variable frequencydivider 24, a fixed frequency divider MV7, the contacts of a relay 25and a xed frequency divider MV5 to a cathode follower power arnplier 26which is connected to the common lead 2T 0f the motor I5.

With these connections, there is applied to the anodes of the amplifiersI2, I3, I'I and I8, from the cathode follower 2t through the lead 2l andthe stator windings I4, I5, I9 and 20, a potential which (l) is ofsquare wave shape and (2) has a. frequency directly proportional to theoutput frequency of the regulated generator 23. l` Such of the thyratronsections of the ampliers I2, I3, II and IB as have a positive pulseapplied from the amplifiers following. the dividers MVz and MVs duringpositive half cycles of the potential applied through the lead 21conduct current during the remainder of these positive half cycles. As aresult, (l) the rotor 2| is stationary when the frequency of the.generator 23 has a predetermined ratio to that of the generator I6,causing unity ratio between the frequencies of the plate and gridpotentials of thyratron sections of amplifiers I2, I3, Il and I8, l(2)the rotor is rotated inv one direction when this ratio increases, and(3) the rotor is rotated in the opposite direction when this ratio defcreases.

- As hereinafter explained, the rotor 2I is mechanically coupled to thefrequency control element ofthe generator 23 so that there is maintainedbetween the output frequencies of the crystal controlled generator Ii)and the regulated generator 23V a ratio predetermined oy the variousfrequency dividers interposed between these generators and the motor I6.

' It has. been indicated that the motor I6 loses 'torque rapidly as itsspeed increases. thaty its torque may bev prevented from decreasing to avalue insufficient to operate properly the frequency control element ofthe regulated generator 23, there are provided `anpair of oscillationgenerators 28v andA 29 havingtheir output' frequenciesV fixed at highand lowlgvalueszby which the maximum operating frequency of the dividervis determined. A l

' As hereinafter explained, one ofthe' differential coilsof the relay 25isv energized-from a; constant current source 30. The other differentialcoil of the relay is energized from the regulatedk generator 23 throughthe divider 24, the divider MV1 and a. self-biased` amplier 3I. So longas the In order output frequency of the generator 23 is within itspredetermined range, the contacts of the relay 25 are in a position toconnect the divider MVr to the divider MVs. When the frequency of thegenerator 23 increases above this range, however, the amplifier 3Ifunctions through the relay 25 to switch the divider MVr; from thedivider MVr to the generator 28-29, and automatically inserting gridresistor 63 into this generator circuit such that it is caused tooscillate at a dennite frequency which is a small percentage higher thanthe frequency which would be fed into MV5 if the oscillation generator23 were exactly at the desired frequency.. Likewise, when the frequencyof the generator 23 decreases below this range the amplifier functionsthrough the relay 25 to switch the divider MV5 from the divider MV'I tothe generator 28-29, and operating with lower frequencyv grid constantsB'I switched in by relays 25. Whenever the frequency of the generator 23is.` brought within the desired range, the relay 25 functions to'connectthe divider MVS to the divider MVr.' Thus the speed of the motor it ismaintained within a range of values. suchv that adequate torque isprovided for relial'ile op-l eration of the frequency control element ofthe regulated generator 23. I

The. circuit connections whereby these results are produced are shown inFig. 2.. In Figs. l and 2 the same reference numerals are. used toindicate corresponding parts. In Fig. 2, one suitable set of values. forthe various parts of the circuit is indicated by legends placed adjacentthe re spective parts. The duotriodes utilized in the frequency dividersmay be of the 2G51 type, for eX- ample.

Itwill be. noted that the frequency dividers are of the two stabilitycondition type previously discussed and that the ampliers I2, I3, Il andI8 each include an input triode 32 and an output tetrode 33 which is ofthe gaseous conduction type. As is well known, current conduction` of`such tetrode is started by the application of a positive pulse to itsgrid while a positive poten-Y tial is applied to its anode, continuesduring the application of such positive potential to itsv anode, and isstopped by the interruption of such positive anode potential.

The frequency divider II is coupled through a capacitor 34 to thejunction point 35 betweenv the common anode resistor 36 and the separateanode resistors 3l and 38 through which operating` potential is appliedfrom a +B lead 39 to the anodes of the divider MV1. With thisconnection, current conduction is shifted from oneY to the other anodevof the divider MV1 each time a negative pulse is applied to the point35.

The divider MV1 has one of its anodes coupled.

through a capacitor 4lito the junction point llly of the divider MVzandthe other of itsanodes coupled through a capacitor t2 to the junctionpoint 63' of the divider MVa. With these connections, there are' appliedto-,the junctionA points il andv 3 negative pulses-.which (l) have afrequency onehalf that of the negative V`pulses applied to thejunction.. point 35 andv (2) are spacedV from one another byequaltimefintervals.

The dividers MVaand MVa operate in the same manner as the divider MV1'.andhave coupled. to each of their anodes through a capacitor 44, Q5, Itor el thegrid of a differentoneofthe triodes 32 of the amplifiers. I2,I3, I'Iand. I8. With these connections there are applied to the grids ofthe triodes 32 of the ampliers I2,v i3, I' and I8 negative pulses whichhave a. frequency one-.-

fourth that of the pulses applied to the junction point and aredisplaced from one another in phase by ninety electrical degrees.

,Such negative pulses function to decrease the current drawn through aresistor 48 by the triode 32, thus applying a more positive potentialthrough a capacitor 49 to the control grid of the gaseous conductiontetrode 33. How these more positive pulses are shifted from one toanother of the tetrodes connected to the stator windings I4, I5, I9 and20 is determined vby the current conductive conditions of the dividersMVz and MVs.

`Thus if the start of current conduction in the upper anodes of thedividers MV2 and MVS, as indicated by the arrows adjacent the individualanode resistors, should occur during the application of a positivepotential through the common lead 21, current is conducted by the statorwindings I4 and I9. This current will endure for the time between theapplication of the positive pulse to the grid of the thyratron 33 andthe loss of the positive voltage on lead 21.

The application of a negative pulse to the junction point 35 shiftscurrent conduction to the lower anode of the divider MV1, thus applyingthrough the capacitor 42 to the junction point 43 a negative pulse bywhich current conduction is shifted from the upper to the lower anode ofthe divider MVa.

.The next negative pulse applied to the junction point 35 functions totransfer current conduction to the upper anode of the divider MV1. Thiscauses a negative pulse to be applied through the capacitor to thejunction point 4|, with the result that current conduction is shiftedfrom the upper to the lower anode of the divider MVz. Each time amultivibrator plate goes from the cut-oil* to the conducting condition,a negative pulse is caused at the grid of amplifier- 32 by thedifferentiating circuit connected thereto. Inversion of this pulse byamplifier 32 places a. positive pulse at the thyratron 33 grid at thistime. Thus the timing of the thyratron grid pulses is controlled by thestandard frequency oscillator.

Whether or not a positive potential is applied to the lead 21 at suchtimes as one of the tetrodes 33 is receiving positive grid pulses isdetermined (1') by the output frequency of the regulated generator 23during the range of frequencies over which the torque of the motor 2D isadequate and (2) by the output frequencies of the generator 28 or 29when the frequency of the regulated motor departs from this range(hereinafter sometimes called the normal range) in either direction.

In Fig. 2, the regulated generator is illustrated as including a triodewhich has operating potential applied to its anode through a ticklercoil 5| and has connected in its grid circuit a coil 52 which isinductively related to the coil 5| and is shunted by a capacitor 53varied by movement ofthe motor shaft 22.

The output frequency of the regulated generator 23 is applied throughthe divider 24 and a capacitor 54 to the junction point 55 of thedivider MV?. The lower anode of the divider MVr, during the normalfrequency range of the generator 23, is connected through the contacts56 and 51 of the relay 25 and a capacitor 58 to the junction point 59 ofthe divider MV5. The upper anode of the divider MVs is connected througha capacitor 6I! to the control grid of a triode 6| which has its outputcircuit coupled through a capacitor cathode follower 26.

62 to the input circuit of the through an inductor with theseconnections, there is applied from the upper terminal of a cathode leadresistor 63 to the lead 21 alternating potential which has a square waveshape and has a frequency directly proportional to the output frequencyof the regulated generator 23. During positive half cycles of thisalternating potential, current is started through the tetrodes 33whenever a positive pulse is applied to their grids and continues untilthe ends of such positive half cycles.

It is desirable that the frequency division ratios be made such that theoutput frequency of the divider MVs is kept near that of the dividersMV1; and MVa. This is so because the speed of the motor |6 is directlyproportional to the difference between the output frequency of thedivider MV5 and that of the dividers MV2 and MVS, and the torque of themotor decreases rapidly with increase in its speed.

Under these conditions, the dividers MVz and MVs function form twobalanced modulators delivering to the windings I4, I5, I 9 and 20 anaverage current of a frequency which is equal to the difference betweenthe output frequency of the cathode follower and that of the dividersMVz and MVs. The rotating magnetic eld of the motor I6 is also of thisdifference frequency, since the plate current of the tetrodes 33 flowsthrough the motor windings. As the difference frequency passes throughzero, the rotational direction of the magnetic field and the armature 2|is reversed. In this way the variable capacitor` 53 controlling thefrequency of the regulated generator 23 is made to rotate in thedirection required to restore the frequency of the generator 23 to thedesired relation with that of the crystal controlled generator I0.

In order to restrict the input frequency of the divider MVs to itsnormal range, two fixed frequency generators 28 and 29 are provided, oneoperating above the input frequency of the dividers MVz and MVa and theother operating below this input frequency. The frequencies of thegenerators 28 and 29 with respect to the frequency of the dividers MV2and MVs determine the maximum deviation of the input frequency of thedivider MVs.

The fixed frequency generators are shown as separate boxes 28 and 29 inFig. 1. In Fig. 2 they are shown as a single generator which includes atriode 64 having its anode coupled through a capacitor 65 to the grid ofa triode B6. The output frequency of this generator is delivered fromthe anode of the triode 66 at one frequency when the relay 25 isoperated to connect the grid of the triode to ground through a resistortact 68 and at another frequency when this grid is grounded through a.resistor 69 and a contact 1|).

Operation of the relay 25 is controlled by a selfbiased amplifier 10'which is connected to the lower anode of the divider tor 1|, a triode12, 14; The cathode of MVv through a capacithe amplifier 10 is groundedvarious elements are so related that the average vider MV7.

The other coil v15| of the relay 25- is supplied with constant currentfrom. the +B lead 39 The relay 25 sur.

through resistors and 8|.

with the cathode follower 26 to 61 and a cona resistor 13 and a,capacitor-- 15 and a capacitor 16, which are shunted by a capacitor 11.The plate cur, v rent of the amplifier 10' ows through one wind-H ing 18of the relay 25. The constants of these 7 adirust'ed'. that the.Ioutput' frequency' of.' theI diividergMVi (which. is. one-halfthatcrtheA variable'l i limiti to the. highest frequency at'which the motorinput voltage; il' may" operate'. This d'oes notlimit the frequencyrange of the oscillator; however; A. particular.l use ofthe inventionis: that of; providing:v a large number of denite frequenciesatlpredetermine dichannelspacings controlled bythe'crystal. oscillationgenerator 23. Such. frequencies'maybe selected byl changing thedivision= ratio ofthe-variable frequency divider: 2G.

What the invention" provides is an improved automatic frequency controlapparatus l` and method of. operationby: which. one crystal controlledgenerator operatingat radio or audio frequencieszmay bemade to; controlwith vcrystal ac'- curacy any desired. number of .output frequencieswhich maybespac'edtso as tovaryoverthe audio and radio' frequencyranges.

What I claimis 1^.. The combination. of'. a crystal. controlled.oscillation. generator; a frequency regulated generator, means forconverting the output of said. crystal controlled generator. topotentials dis.- placed'` from one another in. phaseand havingv afrequency different from; and proportional to that ofV said' crystalcontrolled generator.; means for.l producing an.. alternating,potentialhaving a frequency different: from and' proportional to thatofsaid regulated generator, a polyphasemotor including. windings. which'.have a `common' terminal. and eachv of which. windingshas a separateterminal-z connected to; have appliedl to it a,` different one of saidphase-displaced potentials, Yand meansY connected to` apply saidalternating potential to said. commonV terminal for regulatingl thefrequency of. said regulated. generator.

2. The combination of. a crystal` controlled oscillation generaton, afrequency rregulated gen-- era-tor; meansvforconverting the output ofsaid. crystal controlled generator to: potentials dis-V placed' fronrone another phase and having a of saidy crystal. controlled.generator'means for y frequency different fromandproportional to thatproducing an alternating potential havinga frequency differentifrom andvproportional. to thatof saldi` regulated; generator, means responsiveto.. y

said phase-displaced and said alternating potentialsi`for'regul'atingthef frequency ofi said regu-.-V

lated generator', and relayl means responsive to' deparature cf. theoutput. frequency'of said regu:`

lated. generator. from ai predetermined frequency'y rangei fon xingrlimits: to variation in thefrequency ofzs'aidzalternating potential.

3:. The combination of. a crystal 'controlled' oscillation.. generaton.a .frequency regulated gen-V erator; meansA forl convertingY the outputof'said.'l crystal controlled generator to potentials' displaced from..one another in phase and having a frequency dfferentfrom.andproportional. tozthatoff; said.l crystah. controlledizgenerator;means. fon-l producing an"A alternatingy potential. having a fre``quencyV diierent from and proportional to of said regulated generator, apolyphase motor. in-

cluding windingsy which have.v a common terminal land. each of whichwindings hasV a separate ter.-

minal connected to have applied toit a differenti oneof saidphase-displaced potentials, and means" connected` to apply saidalternating potential. to said common terminal.l for' regulating' the"frequency of said regulated generator.y

4; The' combination of a standard frequency' oscillation. generator, aufrequencyy regulatedv gen-v erator, a plurality' of. frequency' dividersinterconnected to provide. potentials. which are dis placedin phase andhave a. frequency propor tionalto said standardfrequency, means forproducing' an alternating potential .having a free quency different.from and proportional to that of said regulated generator, a polyphasemotor including windings which' have a common terminal4 and. each of;whichwindings has; a. separate terminal connected to have-.applied to ita. different one of said phase-displaced potentials, andmeans connectedvtovapplysaid alternating potential to said. common terminal. forregulating theI frequencyof said regulatedv generator.

5; The combination` of a standard frequency oscillation generator, afrequency regulated gen.- erator, means for converting said standardfre.- quency to potentialswhich. are displaced'iin phase and have.afrequency different from. and' proportional to said standard frequency,a cathode f'ol lower, a plurality of. frequency dividersarranged;v

to be connected between said regulated generator and said cathodefollower for. causing said cathv ode follower to. deliver analternatingcurrent of a frequency proportional tofthatA of said. regulated.generator, a polyphasemotor including' windings which haveV a commonterminal and each of which windings has'- a sepa-rate terminalV connect;ed to have appliedito it a dierent one of said. phase-displacedpotentials, andv means connected to apply saidv alterna-ting potentialto said com.-r rnon terminal for regulating the frequency of said`regulated generator.

6. The combination of a standard frequency oscillation generator, afrequency regulated gen-` erator, meansI for converting saidstandardfrequency to potentials which are displaced in phasel .and have afrequency different from and proportional to said standard frequency,means-in cluding' a `plurality of frequency dividers connectedto said.regulatedgenerator for. producing an. alternating current vof. alfrequencyv proportional.

to that of saidl regulated generator, means re-v parture^ of; the'frequency of said regulated eerr Aerator from-.saidrange.v

7; The combination; of* a standard frequency oscillation generator, afrequency regulatedgen-Y erator, means for converting said standard frequency topotentials whichare displaced in phase.

and. have a frequencyA different from and' proportional. tosaid"standardfrequency, means includinga plurality of frequency dividersconnected tosaid regulatedgenerator'for' producing an alternating.current; of. a frequency' proportional"v to that of said regulatedgenerator; means. re

10 Sponsive to said phase-displaced and alternating REFERENCES CITEDpotentials for regulating the frequency of said The following referencesare o record in the regulated generator over a determined range. me ofthis patent: means for producing two frequencies fixed 1n value at theupper and lower limits of said range, 5 UNITED STATES PATENTS and meansincluding a polarized relay responsive Number Name Date to the outputfrequency of said regulated gen- 2,018,320 Usselman Oct. 29, 1935 eratorfor switching one of said frequency divid- 2,058,114 Usselman Oct, 20,1936 ers from said regulated frequency generator to 2,201,978 BedfordMay 28, 1940 one or the other of said xed frequency producing 10 meansin response to the departure of the frequency of said regulatedgenerator from said range.

ROBERT R. FREAS. JR.

